Removal of solid particulates from fluids--gases and/or liquids--in which the particulates are suspended is commonly done by use of filters. Generally filters are made of porous solid materials in the form of articles or masses with a plurality of pores extending therethrough (which may be interconnected) and having small cross-sectional size or minimum diameter such that the filters are: (1) permeable to the fluids, which flow through the filters from their inlet surface to their outlet surface, and (2) capable of restraining most or all of the particulates, as desired, from passing completely through the filters with the fluid. Such pores constitute what is termed "open porosity" or "accessible porosity". The restrained particulates are collected on the inlet surface and/or within the pores of the filter while the fluids continue to pass through those collected particulates and the filter. The minimum cross-sectional size of each of some or all of the pores can be larger than the size of some or all of the particulates, but only to the extent that significant or desired amounts of the particulates become restrained or collected on and/or within the filters during filtration of the fluids flowing through the filters. As the mass of collected particulates increases, the flow rate of the fluid through the filter usually decreases to an undesirable level. At that point, the filter is either discarded as a disposable/replaceable element or regenerated by suitably removing the collected particles off and/or out of the filter so that it can be reused.
Four general main considerations for useful filters are:
(1) filter efficiency: the amount of suspended particulates of concern in a given volume of fluid that are removed from that volume of fluid as it passes through the filter (usually expressed as a weight percentage of the total particulates of concern originally in that given volume of fluid prior to passing into the filter); PA1 (2) flow rate: the volume of the fluid per unit of time that passes through the filter and collected particulates or, in a closed continuous feed system, the back pressure or increased pressure created in such system upstream from the filter by the presence of the filter and particulate collected thereon in comparison to what the pressure therein would have been in the absence of the filter; PA1 (3) continuous operating time: the cummulative time of continued service of the filter before filter efficiency and/or flow rate/back pressure become unacceptable so as to necessitate replacement and/or regeneration of the filter; and PA1 (4) compact structure: smallest space-saving volume and configuration of the filter for attaining the best combination of filter efficiency, flow rate/back pressure nd continuous operating time.
For filtration of fluids at elevated temperatures, consideration must also be given to the filters having adequate mechanical and chemical durability under the prevailing conditions of temperature within the filter and chemical reactivity of the fluids and suspended particulates coming into contact with the filter.
The considerations noted above, especially the four general main ones, appear to be accommodated in varying degrees, but in less than fully satisfactory ways, by the following examples of prior art filters or incomplete filter suggestions:
U.S. Pat. Nos. 2,884,091, 2,952,333 and 3,242,649 illustrate filters of the type made of pleated thin porous sheets of filter material whose layers are interleaved with corrugated or crimped spacers with the parallel corrugations or crimps thereof extending substantially perpendicular to the folds of the pleated sheets. In essence, fluid enters a complete layer or column of cells defined by a spacer and then passes through only the filter sheets on each side thereof (but not through corrugation or crimp segments of a spacer separating adjacent cells in that spacer) to effect filtration. Moreover, the corrugations involve cell-like passages whose transverse cross-sections have sinusoidal geometric shapes having small angle "corners" of substantially less than 30.degree..
British patent specification No. 848,129 shows another form of the pleated-type filters wherein, instead of being interleaved with corrugated spacers, the thin porous sheets of filter material are impressed with spacer dimples to maintain spacing between the pleats.
U.S. Pat. No. 3,346,121 discloses thin-porous-walled honeycomb filters of corrugated layer structure having crosswise oppositely indented portions that block end portions of the channels or passages in an alternating pattern within each layer (but not necessarily from layer to layer) to cause fluid therein to pass through the porous walls to effect filtration of the fluid. The corrugation pattern is such that the channels or cells have transverse, cross-sectional, geometric shapes with numerous instances of corners formed by small angles substantially less than 30.degree.. Moreover, the layered structure involves numerous portions, where the layers adjoin each other, which are of double and sometimes triple layer or wall thickness.
U.S. Pat. No. 3,533,753 describes catalyst bodies with layered networks of intersecting "capillary" channels which can function as a filter body for combustion exhaust gas dust or sedimented particles, which can be diesel engine exhaust soot or particulates as noted in U.S. Pat. No. 4,054,417.
U.S. Pat. No. 3,637,353 discloses a tubular packed bed of granular catalyst with fluid-flow interstices for filtering particulates from exhaust gases generated by diesel engines.
U.S. Pat. No. 4,054,417 also suggests making the disclosed diesel exhaust filters of known materials used in heat exchangers for turbine engines or in monolith catalytic converters for automotive vehicles (e.g. as disclosed in U.S. Pat. No. 3,122,184 as a corrugated structure and in U.S. Pat. No. 3,790,654 as an extruded structure) as alternatives to and in a manner similar to the material in U.S. Pat. No. 3,533,753 (i.e. with fluid flow passing into, through and out of every channel).
Research report EPA-600/2-77-056 of the U.S. Environmental Protection Agency suggests that several commercially available thin-porous-walled ceramic monoliths of honeycomb appearance, both corrugated and extruded, are potential filters for diesel exhaust particulates. However, the only illustrated arrangement given therefor is the alternate layer cross-flow design of a corrugated monolith, with small corners less than 30.degree. in transverse cross section, wherein the exhaust gas passes through only those thin walls between layers of cells or passages. This report also suggests the suitability of porous bonded masses of ceramic fibers for filters of diesel exhaust particulates.
British patent specification No. 1,440,184 discloses that porous bonded sheets of refractory metal oxide fibers can be formed into corrugated or embossed honeycomb structures for use in filtration of hot waste gases containing particulate matter and of molten metal prior to casting. As in cases noted above, the transverse cross-section of the corrugated embossed or structures contain numerous small angle corners much less than 30.degree..
U.S. Pat. Nos. 4,041,591 and 4,041,592 disclose thin-walled, honeycombed, multiple-fluid-flow-path bodies with all cells or passages parallel such that fluid entering each of the passages can continue through and pass out of the open exit end thereof without passing through any cell wall. Alternate selected columns or layers of cells have their ends sealed for advantageous separate manifolding to fluid conduits. An optional use indicated for these bodies is in filtration and osmosis when porous materials are used to form the honeycombed body so that some of the fluid flowing in a first set of cells can pass into an adjacent alternate set of cells through the thin porous walls between them while a remaining portion of the fluid with a higher concentration of an undesirable or separable constituent can continue through and pass out of the open exit end of the first sets of cells. Examples of the latter use are reverse osmosis filtration and ultrafiltration of saline or impure water to produce potable or purified water, in which cases the surfaces of the porous walls defining the first set of cells are lined with suitable selectively permeable membranes.